Nanoimprint lithography for high-throughput fabrication of metasurfaces

Metasurfaces are composed of periodic subwavelength nanostructures and exhibit optical properties that are not found in nature.They have been widely investigated for optical applications such as holograms,wavefront shaping,and structural color printing,however,electron-beam lithography is not suitable to produce large-area metasurfaces because of the high fabrication cost and low productivity.Although alternative optical technologies,such as holographic lithography and plasmonic lithography,can overcome these drawbacks,such methods are still constrained by the optical diffraction limit.To break through this fundamental problem,mechanical nanopatteming processes have been actively studied in many fields,with nanoimprint lithography(NIL)coming to the forefront.Since NIL replicates the nanopattem of the mold regardless of the diffraction limit,NIL can achieve sufficiently high productivity and patterning resolution,giving rise to an explosive development in the fabrication of metasurfaces.In this review,we focus on various NIL technologies for the manufacturing of metasurfaces.First,we briefly describe conventional NIL and then present various NIL methods for the scalable fabrication of metasurfaces.We also discuss recent applications of NIL in the realization of metasurfaces.Finally,we conclude with an outlook on each method and suggest perspectives for future research on the high-throughput fabrication of active metasurfaces.

On-demand design of spectrally sensitive multiband absorbers using an artificial neural network

We report an approach assisted by deep learning to design spectrally sensitive multiband absorbers that work in the visible range.We propose a five-layered metal-insulator-metal grating structure composed of aluminum and silicon dioxide,and we design its structural parameters by using an artificial neural network(ANN).For a spectrally sensitive design,spectral information of resonant wavelengths is additionally provided as input as well as the reflection spectrum.The ANN facilitates highly robust design of a grating structure that has an average mean squared error(MSE)of 0.023.The optical properties of the designed structures are validated using electromagnetic simulations and experiments.Analysis of design results for gradually changing target wavelengths of input shows that the trained ANN can learn physical knowledge from data.We also propose a method to reduce the size of the ANN by exploiting observations of the trained ANN for practical applications.Our design method can also be applied to design various nanophotonic structures that are particularly sensitive to resonant wavelengths,such as spectroscopic detection and multi-color applications.

Giant chiro-optical responses in multipolar-resonances-based single-layer dielectric metasurfaces

Chiro-optical effects offer a wide range of potential applications in nanophotonics,such as advanced imaging and molecular sensing and separation.Flat single-layer metasurfaces composed of subwavelength meta-atoms have gained significant attention due to their exceptional characteristics in light–matter interactions.Although metasurface-based devices have manipulated electromagnetic waves,the compact on-chip realization of giant chiro-optical effects remains a challenge at optical frequencies.In this work,we experimentally and numerically demonstrate an all-dielectric metasurface to realize large chiro-optical effects in the visible regime.Notably,the proposed strategy of utilizing achiral nanofins instead of conventional chiral structures provides an extra degree of design freedom.The mutual coupling between carefully engineered nanofins produces constructive and destructive interference,leading to the asymmetric transmission of 70%and average circular dichroism exceeding 60%.We investigate the underlying mechanism behind the chiro-optical effects using the theory of multipolar decomposition.The proposed design mechanism maximizes the chiro-optical response through a single-layer metasurface with potential applications in high-efficiency integrated ultrathin polarization rotators and shapers,chiral polarizers for optical displays,chiral beam splitters,and chiral sensors.